The present invention relates to a method of cleaning gas mixtures from a urea plant, particularly such mixtures containing dust, and especially off-gases from prilling towers or granulation units and where the gases are scrubbed by an aqueous solution.
Urea prills or granules are produced from highly concentrated urea solutions or melts and treated in a prilling tower or granulation unit where air is used for cooling the product. The air drawn from these units must be cleaned before it is released into the atmosphere because it will contain urea dust, ammonia and different other decomposition products of urea.
The air drawn from the urea plant is scrubbed by an aqueous solution in order to reduce air pollution from the plant. This aqueous solution usually circulates in the scrubber until it has the desired concentration of urea, whereupon it is returned to the process for reclaiming urea. There are known several types of such scrubbers.
From DE-application No. 2,513,925 it is known to employ a venturi scrubber, in which water is mixed with the off-gases before they are conducted into the venturi scrubber. Water is primarily mixed with the off-gases in order to reduce plugging of the venturi nozzles by urea dust.
During washing of the off-gases from a prilling tower with a circulating aqueous urea solution, problems have been experienced with precipitation giving accretion and plugging of the scrubber. The precipitation resulted in increasingly thick accretions in the scrubber. Finally, dispersed mud sedimented in the tanks and pipes and a hard brittle coating occurred on all surfaces that came into contact with the washing solution. The nozzles for spraying liquid into the multiventuri were plugged and the ability of the unit to separate dust from water was soon strongly reduced.
The precipitates and the accretions were insoluble in water, and even when the supply of water was increased this did not help. Mechanical cleaning of the unit by scraping or the like of the deposits was accordingly very laborious and expensive.